A glow discharge lamp includes a light transmitting lamp envelope containing a noble gas and mercury. The lamp envelope typically has the shape of a conventional incandescent lamp with a bulbous portion and a neck portion. A phosphor coating is applied to the inner surface of the lamp envelope. One or more cathodes and one or more anodes are mounted within the lamp envelope. Upon heating of the electrodes and application of a voltage between the electrodes, a glow discharge is formed within the lamp envelope. The phosphor coating absorbs ultraviolet radiation that is emitted by the glow discharge and emits visible light. Examples of glow discharge lamps are disclosed in U.S. Pat. No. 3,814,971 issued June 4, 1974 to Bhattacharya, U.S. Pat. No. 2,067,129 issued Jan. 5, 1937 to Marden, U.S. Pat. No. 4,408,141 issued Oct. 4, 1983 to Byszewski et al and U.S. Pat. No. 4,751,435 issued June 14, 1988 to Roche et al.
One disadvantage of a glow discharge lamp used for general purpose illumination is that when power is applied to the lamp, the lamp does not provide illumination instantaneously. Starting for typical glow discharge lamps can take from three to ten seconds. A bimetal switch or other thermally-sensitive switch is connected in series between the anode and cathode. When the power switch is closed, the electrodes and an emissive coating on the electrodes are heated, thereby causing emission of electrons. A gas breakdown occurs, and ultraviolet radiation is emitted by the glow discharge. After a glow discharge is established, the bimetal switch opens. In a standard fluorescent lamp, this sequence of events occurs in less than one second. However, in a glow discharge lamp used for general purpose illumination, starting takes considerably longer because more massive, low electrical resistance electrodes are used to supply relatively high discharge currents. It is known that smaller, higher resistance electrodes will start the glow discharge lamp more rapidly. However, this configuration is undesirable because the smaller electrodes overheat during steady state operation. Furthermore, the smaller electrodes do not hold sufficient emissive material to insure a long lamp life.
Various approaches have been proposed for overcoming the slow starting of glow discharge lamps. One technique involves the application of a short duration, high current pulse through the cathode, thereby causing rapid heating. This technique requires a relatively complicated starting device. Another technique, disclosed in U.S. Pat. No. 3,215,881 issued Nov. 2, 1965 to Waymouth, involves the use of a small electrode for starting and a large electrode for steady state operation. This technique also complicates the lamp construction.
A fluorescent lamp having incandescent filaments connected in parallel with each electrode is disclosed in U.S. Pat. No. 3,521,122, issued July 21, 1970 to Peek, Jr. Since the incandescent filaments emit light during steady state operation, the efficiency of the lamp is reduced. Peek, Jr. states that the incandescent filaments can be switched out of the circuit during normal operation of the lamp.
A germicidal lamp wherein two electrodes coated with an emissive material are connected in series is disclosed in U.S. Pat. No. 2,560,933 issued July 17, 1951 to Chun et al. Although current flows through the electrodes during starting, the current is insufficient to cause emission of significant visible light from the electrodes.
U.S. Pat. No. 3,849,699, issued Nov. 19, 1974 to Roche, discloses a self-igniting, fluorescent lamp including an ignition coil which extends between the ends of the lamp and is coated with an electron emissive material. The ignition coil does not emit significant visible light.
U.S. Pat. No. 3,878,416, issued Apr. 15, 1975 to Roche et al, discloses a fluorescent lamp having an ignition coil which extends between the ends of the lamp and a separate incandescent lamp outside the fluorescent lamp envelope which acts as a ballast for the fluorescent lamp. The ignition coil does not emit significant visible light. The incandescent lamp is illuminated during steady state operation and thereby reduces the operating efficiency of the assembly. A lamp assembly including a fluorescent lamp and a separate incandescent lamp which acts as a ballast for the fluorescent lamp is also disclosed in U.S. Pat. No. 4,268,780, issued May 19, 1981 to Roche et al. Since the incandescent lamp is on during steady state operation, the efficiency of the assembly is relatively low.
U.S. Pat. No. 1,925,648, issued Sept. 5, 1933 to Spanner et al, discloses a gas discharge device having an elongated envelope with electrodes mounted at each end and a helix which extends between the ends of the lamp envelope. The helix is coated with an electron emissive material and does not emit substantial visible light.
U.S. Pat. No. 4,329,622, issued May 11, 1982 to Corona et al, discloses a low pressure gas discharge lamp containing a pair of high power, low emissivity filaments and a pair of high emissivity filaments. The low emissivity filaments provide additional light to compensate for the reduced illumination profile near the ends of the lamp. Since the filaments remain on during steady state operation, the efficiency of the lamp is reduced by the filaments.
All of the known arrangements for starting glow discharge lamps or other fluorescent lamps have one or more disadvantages, including increased cost and complexity, failure to provide visible light immediately when power is applied to the lamp and reduced efficiency during steady state operation. It is desirable to provide a glow discharge lamp which overcomes these disadvantages.
It is a general object of the present invention to provide improved glow discharge lamps.
It is another object of the present invention to provide glow discharge lamps which emit visible light immediately upon application of power.
It is a further object of the present invention to provide glow discharge lamps which have high operating efficiencies.
It is yet another object of the present invention to provide glow discharge lamps which are simple in construction and low in cost.